Quantity Pulldown / Bulk Ordering Feature

Can we make the quantity a pulldown or make quantity required by 100's?

We only want to allow orders in bulk... by the 100's:

100
200
300
400
500
600
and so on...

Yes, that's possible as explained on that thread:
www.hikashop.com/en/forum/4-how-to/15254...uantity-ordered.html

I have one product that needs a different pulldown quantity? is this possible?

Meaning ALL products use: 100/200/300/400
EXCEPT ONE product needs: 5/10/15/20

Yes, such overrides allow to do that but it's more complex as you need to write PHP code to change the quantity values based on the product where you are.

any advice on where to start with this?

It's the same as the code of the thread I already gave a link to.
Simply add an if/else based on the product where you are. For example: if you want a specific dropdown for the product with the id 18 :

<?php
function hikashop_quantity_render($html,$i,$max_quantity,$min_quantity){
if(hikashop_getCID('product_id')==18){
return '<select id="hikashop_product_quantity_field_'.$i.'" name="quantity"><option value="25">25</option><option value="50">50</option><option value="75">75</option><option value="100">100</option></select>'.$html;
}else{
return '<select id="hikashop_product_quantity_field_'.$i.'" name="quantity"><option value="50">50</option><option value="100">100</option><option value="200">200</option><option value="500">500</option></select>'.$html;
}
}